Temperature compensated current source

ABSTRACT

A current source which is substantially independent of variations of temperature. Thus the current source may be made either to have a linear dependence upon changes of temperature or, by the simple addition of a resistor, may be made substantially independent of temperature variations. Since the current source consists only of transistors of one conductivity type and resistors, it is ideally suited for manufacture in the form of a monolithic integrated circuit. The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568.

United States Patent [72] Inventor [21 Appl. No. [22] Filed [45]Patented [73] Assignee [54] TEMPERATURE COMPENSATED CURRENT SOURCE 3Claims, 3 Drawing Figs.

[52] US. Cl. 307/270, 307/297, 323/4, 328/172 [51] Int. Cl 605i H40 [50]Field of Search 307/270, 297; 328/172, 183, 184; 323/4, 20

[56] References Cited 1 UNITED STATES PATENTS 3,069,617 12/ 1962 Mohler307/297 3,105,187 9/1963 Schauwecker 307/297 3,201,606 8/ 1965 Mamon307/297 3,300,658 1/1967 Slusher et a1. 307/297 Voltage Regulator3,317,818 5/1967 Beyer 307/297 3,435,257 3/1969 Lawrie, Jr 307/2912,816,964 1/1957 Giacoletto 330/24X 2,991,407 7/1961 Murphy 323/43,246,233 4/1966 Herz 323/4 3,271,528 9/1966 Vallese 330/30 3,383,6125/1968 l-larwood 330/30 Primary ExaminerDonald D. Forrer AssistantExaminerR. C. Woodbridge Attorney-George C. Thompson, Jr.

ABSTRACT: A current source which is substantially indepen dent ofvariations of temperature. Thus the current source may be made either tohave a linear dependence upon changes of temperature or, by the simpleaddition of a resistor, may be made substantially independent oftemperature variations. Since the current source consists only oftransistors of one conductivity type and resistors, it is ideally suitedfor manufacture in the form of a monolithic integrated circuit. Theinvention described herein was made in the performance of work under aNASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of 1958, Public Law 85668.

F lg. 1

4| 42 a 43 45 L I 4 I Drift Offset Drift Offset Voltage 0 Control 1Control 1 Control 1 Control Regulator Fig. 2.

Voltage Regulator David R. Breuer INVENTOR.

ATTORNEY.

TEMPERATURE COMPENSATED CURRENT SOURCE BACKGROUND OF THE INVENTION Thisinvention relates generally to current sources, and particularly to acurrent source which may be made either substantially temperatureindependent or linearly dependent upon temperature variations.

For many purposes current sources are needed where the output currentsubstantially does not vary with temperature over a wide temperaturerange. For example, in order to have a precision analog circuit suchcurrent sources may be needed as the energy sources of the amplifiers.Also amplifier drift may be compensated by utilizing a current sourcehaving a known linear dependency on temperature.

It is accordingly an object of the present invention to provide acurrent source, the output current of which varies linearly withvariations of temperature.

Another object of the invention is to provide a current source of thecharacter discussed which may be made substantially independent oftemperature variations over a wide temperature range by the simpleaddition of a resistor.

A further object of the present invention is to provide a temperaturestable current source consisting of resistors and transistors so that itmay readily be manufactured in the form of a monolithic integratedcircuit.

SUMMARY OF THE INVENTION In accordance with the present invention thereis provided a current source for providing an output current'which isapproximately independent of temperature variations. Thus the outputcurrent may be made either independent of or linearly dependent upontemperature changes. Such a current source is ideally suited tocompensate the inherent temperature drift of a direct-coupleddifferential-input amplifier.

The current source comprises two transistors which are interconnected insuch a manner as to provide a feedback loop. To this end the base of thefirst transistor is connected directly to the collector of the secondtransistor, while the emitter of the first transistor is connecteddirectly to the base of the second transistor. A first resistor isconnected between one terminal of a source of regulated voltage and thecollector of the second transistor as well as the base of the firsttransistor. Finally, a second resistor is provided between the junctionpoint of the emitter of the first transistor and the base of the secondtransistor on the one hand, and the other terminal of the voltage sourceon the other hand. Further, the emitter of the second transistor is alsoconnected to the other terminal of the voltage source.

Accordingly an output current is available from the collector of thefirst transistor which is substantially linearly dependent ontemperature. This is true as long as the reciprocal value of the beta ofeach transistor is substantially negligible compared to 1.

In order to make the output current substantially independent oftemperature variations, all that is necessary is to connect a thirdresistor between the emitter of the second transistor and the otherterminal of the voltage regulator.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, aswell as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a circuit diagram, partly inblock form, of a differential-input amplifier which may utilize thecurrent sources of the present invention;

FIG. 2 is a circuit diagram of a current source embodying the presentinvention for delivering an output current whichv is linearly dependenton temperature variations and which may be used with the amplifier ofFIG. I; and

FIG. 3 is a circuit diagram of a modified current source in accordancewith the present invention for delivering an output current which issubstantially independent of temperature variations and which may alsobe used with the amplifier of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing,there is illustrated in FIG. l a differential-input amplifier. Thisamplifier is claimed in the copending application of the presentinventor, entitled Direct-Coupled Differential-Input Amplifier," filedconcurrently herewith and assigned to the assignee of the presentinvention. The current sources of FIGS. 2 and 3 which embody the presentinvention may be used with the amplifier of FIG. ll, although thisamplifier may use other previously known current sources.

The differential amplifier of FIG. I has a pair of input terminals l0and 11 from which the differential-mode input signal is available. Asexplained before, the differential-mode signal is the desired signalbetween terminals 10 and 11 and preferably is an input voltage. Theundesired common-mode component is that component which is common to thetwo input terminals 10 and 11. Typically the common-mode component maybe on the order of volts, while the desired differential-mode signal maybe as low as microvolts.

The input signal is impressed on a pair of input transistors 12 and 14which, as shown may be of the NPN type. In any case they should both beof the same conductivity type. The input signals are impressed on thetwo bases of the input transistors 12 and 14, and accordingly the inputterminals 10 and 11 are respectively connected to the two bases.

The output signal is obtained from the two collectors of the two inputtransistors 12 and 14 and is directly impressed on the two inputterminals of an amplifier 15 which may be called the differential-modeamplifier. The differential-mode amplifier 15 has a single-ended outputterminal 17 on which the amplified differential-mode signal is obtained.

A feedback connection 18 is provided between the output terminal 17 ofthe differential-mode amplifier 15 and ground and includes a resistivefeedback network 20 consisting of resistors 21, 22, 23 and 24 connectedin series with each other and between the amplifier output terminal 17and ground. The feedback connection is completed to the emitters of thetwo input transistors 12 and 14. This emitter feedback connection isimportant for the operation of the amplifier of the invention.Accordingly, the junction point between resistors 21 and 22 is connectedto the emitter of transistor 12. Similarly, the junction point betweenresistors 23 and 24 is connected to the emitter of transistor 14.

Further in accordance with the present invention, there is provided ahigh-gain common-mode feedback loop including a stabilizing amplifier26. The amplifier 26 has one input terminal 27 connected to a point ofreference potential shown as E This may be any suitable stable voltageincluding ground. The other input terminal 28 of the stabilizingamplifier 26 is connected to an output terminal 30 of thedifl'erential-made amplifler I 5. From this output terminal 30 there isavailable the common-mode signal. This may be considered proportional tothe average collector currents of the input transistors I 2 and 14. Thestabilizing amplifier 26 accordingly develops an output current at itsoutput terminal 31 having a magnitude which depends on the difference ofthe voltages at its input terminals 27 and 28. In other words, itdevelops an output current which varies with variations of thecommon-mode signal.

In order to complete the common-mode feedback loop, the output terminal31 of the stabilizing amplifier 26 is connected to the junction betweenresistors 22 and 23. Thus the emitter currents of the input transistors12 and 14 are held constant under conditions of changing currents in theresistive feedback network 20. A positive voltage source 33 may beconnected to the amplifier output terminal 31 by a resistor 34 as shown.

As stated above, it is the function of the stabilizing amplifier 26 todevelop an output current of a magnitude and direction to compensate forvariations of the common-mode voltage component at the input terminalsand 11. Stated another way, the common-mode feedback loop including thestabilizing amplifier 26 controls the operating bias levels for thetransistors 12 and 14 so that the overall gain of the differential-modeamplifier remains constant in spite of variations of the common-modeinput voltage component.

The amplifier circuit as described so far has a differential input and asingle-ended output and affords a high rejection of the common-modecomponent. The common-mode rejection may be at least as large as 100 db.(decibels). On the other hand, the amplifier circuit as described so fardoes not compensate for possible temperature drift nor for inherentvoltage offset. Thus the amplifier is operative if it is maintained at aconstant temperature and if the inherent voltage offset can be minimizedfor any particular application. However, if the ambient temperaturevaries over a wide range, say, from -40 C. (centigrade) to +100 C., someprovision must be made to prevent output signal variations withtemperature variations.

In accordance with the present invention, this is effected by theprovision of a drift control circuit 40 and an offset control circuit 41for supplying current to the emitter of transistor 14, and a similardrift control circuit 42 and offset control circuit 43 for supplyingoperating current to the input transistor 12. The two drift controlcircuits 40 and 42 are designed to develop an output current whichvaries linearly with temperature variations. Hence they compensate fortemperature drifts. On the other hand, the offset control circuits 41and 43 control current offset and deliver an output current which issubstantially independent of temperature variations.

To explain the functions of the drift control circuits 40,42 and of theoffset control circuits 41,43, we may consider a plot of the voltage asa function of temperature variations. It is the function of the driftcontrol circuits to control the slope of the voltage-versus-temperaturecurve. On the other hand, the offset control circuit controls theabsolute value of the voltage. In other words, this makes it possible tohave a zero output voltage for a zero input voltage, provided the slopeof the curve has been made 0 by proper control of the drift control circuits.

The control circuits 40 through 43 may each have an input connected to avoltage regulator 45 for supplying thereto a regulated input voltage. Asshown, the other terminal of the voltage regulator 45 and of the controlcircuits 40 to 43 may be grounded. The drift and offset control circuits40 and 41 have an output lead 46 connected to the emitter of transistor14. Similarly, the drift and offset control circuits 42 and 43 have anoutput lead 47 connected to the emitter of input transistor 12.

It has been found that by adjusting the relative components of thecurrents delivered by the drift and offset control circuits 40 and 41,or 42 and 43, the amplifier can be so adjusted that thedifferential-mode signal obtained at output terminal 17 is renderedsubstantially independent of temperature variations. Also thedifferential-mode signal at output terminal 17 may be made zero for azero input signal. Actually it has been found that the temperature driftfor a temperature range between -40 C. and +100 C. may be maintained tobe less than 0.05 uv/ C. (uv indicating microvolts).

It will also be appreciated that current sources such as the driftcontrol circuits 40 and 42 which deliver an output current linearlydependent on temperature variations are well known. Similarly, offsetcontrol circuits such as 41 and 43 which deliver an output currentsubstantially independent of temperature variations are also well known.However, it has been found that the circuits illustrated in FIGS. 2 and3 are particularly suitable for this purpose.

The following formula shows why the output voltage of the amplifier ofFIG. 1 can be made independent of temperature. In the followingformulav, is the output voltage obtained at output terminal 17. 1,, indicatesthe current flowing in the lead 47. Similarly, 1 is the current flowingin lead 46. Furthermore, W1 is the input voltage at input terminal 10,while vB is the other input voltage at the terminal 11. Finally, R isthe combined resistance of resistors 22 and 23, and R is the combinedresistance of resistors 21 and 24. The following formula is thusobtained for v,,:

0 (IA-I13) 2+ iam) Formula (1) shows that v, may be made temperatureindependent by a proper adjustment of the currents 1,, and I becauseboth currents contain separately a temperature independent and alinearly temperature dependent component. Thus by adjustment of therelative currents obtained from the drift control circuits 40 and 42compared to the currents obtained from the offset control circuits 41and 43, the output voltage v, may be made temperature independent. Thevoltage v may also be made zero for a zero input signal, that is, whenvA equals vB. This is the function of the offset control.

Referring now to FIG. 2, there is illustrated a transistor circuit whichmay be considered as a current source delivering an output current whichis a linear function of the tempera ture. Accordingly the circuit ofFIG. 2 may be used to obtain the drift control shown at 40 and 42 inFIG. 1. The circuit of FIG. 2 includes a pair of transistors 50 and 51which may be of the NPN type as shown. In any case they should both beof the same conductivity type.

The two transistors 50 and 51 are connected directly to each other toform a feedback loop. Accordingly the collector of transistor 51 isconnected directly to the base of transistor 50. Similarly, the emitterof transistor 50 is directly connected to the base of transistor 51.

As mentioned before, the transistors are preferably supplied with aregulated voltage from the voltage regulator 45. Thus the positiveterminal of the voltage regulator may be connected by a resistor 52 tothe base of transistor 50 and the collector of transistor 51. Thenegative terminal of the voltage regulator 45 is directly connected tothe emitter of transistor 51. The emitter of transistor 50 and the baseof transistor 51 are connected through a resistor 53 to the emitter oftransistor 51 and to the negative terminal of the voltage regulator.Finally, the output current is obtainable from the output terminal 54connected to the collector of transistor 50.

With the voltages of the voltage regulator and the conductivity types ofthe two transistors as shown, current flows into the collector oftransistor 50, as shown by the arrow 55. There is further a closedcurrent loop which may be traced from the collector of transistor 51 tothe base of transistor 50, as shown by arrow 56, and then through theemitter of transistor 50 and the base of transistor 51, as shown by thearrow 57.

It will be realized that the alpha of a junction transistor generallyvaries nonlinearly with temperature. The alpha of a transistor isdefined as the variation of the collector current with variations of theemitter current, the voltage between collector and emitter beingmaintained constant. Similarly, the beta of a transistor is defined asthe variation of the emitter current with variations of the basecurrent, the voltage between collector and base being maintainedconstant.

For further discussion it will be assumed that the two transistors 50and 51 are closely matched with respect to their alphas and betas. Inother words, we have to assume that the collector currents of the twotransistors, as well as the betas of the two transistors, areapproximately equal. In that case their base currents will also beequal. Accordingly, if the collector current of transistor 51 isapproximately constant with temperature, the change of the voltage dropbetween base and emitter of transistor 51 will be extremely linear withtempera ture. Accordingly, the current through resistor 53 equals theoutput current which is shown by the arrow 55. The reason is that "equalcurrentsfiOWtls'shoWn by "the arrows 56 and 57. Therefore the outputcurrent is also a very linear function of temperature.

Since equal currents flow in the directions shown by arrows 56 and 57,it will be apparent that the output current also flows through resistor53. Therefore the magnitude of the output current may be controlled byvarying the resistance of the resistor 53 as shown. This is the mannerin which the drift control circuits 40 and 42 may be adjusted.

The circuit of FIG. 2 may be mathematically analyzed and the followingformula may be obtained:

In the above formula, I is the output current, that is, the currentflowing through the output terminal 54. R is the resistance of resistor53. B, is the beta of transistor 51. B is the beta of transistor 50. Ris the resistance of resistor 52. E is the output voltage developed byvoltage regulator 45. V is the base-emitter voltage of transistor 51,and V is the baseemitter voltage of transitor 50.

Assuming that [/13 may be neglected with respect to 1, and that 1 /]3may be neglected with respect to l, formula (2) may be simplified asfollows:

This condition may be expressed another way, namely, if [3, and B iseach larger than 100, the reciprocal of beta certainly can be neglectedwith respect to 1.

Formula (3) shows that I, the output current, is a linear function oftemperature because V is a linear function of temperature, as are thebetas of the transistors.

Accordingly the circuit of FIG. 2 will deliver an output current whichis a linear function of temperature under the conditions referred toabove. The circuit may be used for the drift control circuits 40 and 42.Also the magnitude of the output current may be adjusted by adjustingthe resistance of resistor 53.

By a simple modification the circuit of FIG. 2 may be made to deliver anoutput current which is substantially independent of temperature. Thisis illustrated in FIG. 3. The circuit of FIG. 3 is identical to that ofFIG. 2 except that a resistor 60 is connected between the emitter oftransistor 51- and the negative terminal of the voltage regulator 45.

In the circuit of FIG. 3 there is a 100 percent feedback loop betweenthe two transistors 50 and 51. Accordingly the voltage across resistor53 is stabilized with temperature if the ratio of the resistance ofresistor 60 to that of resistor 52 is equal to the alpha of transistor51 which is in the neighborhood of one. It should also be assumed thatthe voltage gain of the network consisting of transistor 51, resistor 52and resistor 60 is unity.

It has already been explained that the output current flow throughoutput terminal 54 is equal to the current through resistor 53.Therefore it will be apparent that the output current is constant withtemperature and is inversely proportional to the resistance of resistor53. In other words, the magnitude of the output current may again beadjusted by an adustment of the resistance of resistor 53. As pointedout before, this affords a simple manner in which the output current ofthe offset control circuits 41 and 43 may be adjusted.

The temperature independence of the output current of the voltage sourceof FIG. 3 may also be shown mathematically asfollows. Assuming that theresistances of resistors 52 and 60 are equal, the output current I isdetermined by the following formula:

This formula (4) may again be simplified provided the previousassumption is true that the reciprocals of the betas of the twotransistors may be neglected compared to 1; or, put in other words, thatthe betas of the two transistors should be no less than 100.Furthermore, we assume that the input voltage E is greater than 2V or 2VThe reason for that is that the input voltage should be larger than thevoltage drops between the base and emitters of the two transistorsconnected in cascade. Practically, the input voltage E should be greaterthan 10V or l0V With these assumptions, formula (4) may be simplified asfollows:

Formula (5) shows that the output current is indeed independent oftemperature because both E, the input voltage, and R, the resistance ofresistor 53, are assumed to be temperature independent.

It will be understood that the circuit specifications of the voltagesource of FIG. 2 may vary according to the design for any particularapplication. However, the following circuit specifications have beenfound to be suitable for use with a direct-coupled differential inputamplifier of the type shown in FIG. 1:

Resistor 52 130,000 ohms Resistor 53 12,000 ohms Transistor 50 Type 2N9l 8 and Transistor 51 Type 2N9 l 8 With the circuit specifications givenabove, the current source of FIG. 2 develops an output current which isa linear function of temperature and which, when plotted, deviates lessthan 0.1 percent from a straight line over a temperature range between-40 C. and +l00 C.

For the current source of FIG. 3, the same transistor types may be usedand the following resistance values been found to be suitable forapplication as a drift control circuit of the type shown in FIG. 1:

Resistor 52 47,000 ohms Resistor 60 47,000 ohms Resistor 53 38,000 ohmsWith the above circuit specifications, the current source of FIG. 3 issubstantially independent of temperature within a range of 40 C. to C.and is stable within 10 to 100 parts per million/ C.

As pointed out before, the circuits of FIGS. 2 and 3 are particularlysuitable for use in the form of a monolithic integrated circuit. In thatcase the characteristics of transistors 50 and 51 will be very closelymatched. It is also feasible to obtain at least a small adustment of theresistor 53. Thus assuming that the resistors are cermet resistors whichmay consist, for example, of chromium with silicon monoxide having athickness on the order of 300 A. (Angstrom units), the resistance may beadjusted after the circuit has been made, for example, by heating thecermet material of a particular resistor such as 53.

There has thus been disclosed a current source which consists only oftransistors of one conductivity type and resistors. Accordingly thecurrent source is ideally suited for manufacture in the form of amonolithic integrated circuit. The current source may be made to deliveran output current which is very linear with variations of temperatureover a wide temperature range. By the mere addition of a resistor, thecurrent source may be made to be substantially independent oftemperature within 10 to 100 parts per million/ C. Furthermore, themagnitude of the output current may be controlled by adjustment of oneof the resistors of the circuit.

I claim:

1. A current source for providing an output current which is linearlydependent upon temperature variations, said source comprising:

a. a first and a second transistor, said transistors being'of the sameconductivity type and having substantially equal alphas and betas;

b. said first transistor having its base connected directly to thecollector of said second transistor;

. said first transistor having its emitter connected directly to thebase of said second transistor;

. a source of regulated voltage;

. a first resistor connected between one of the terminals of saidvoltage source and the collector of said second transistor; a secondresistor connected between the junction point of the emitter of saidfirst transistor and the base of said second transistor, and the otherterminal of said voltage source, the emitter of said second transistorbeing coupled to said other terminal of said voltage source; and

g. means isolated from said source and connected to the collector ofsaid first transistor for deriving an output current, said outputcurrent being substantially a linear function of temperature providedthe reciprocal value of the beta of each of said transistors issubstantially negligible compared to one.

2. A current source as defined in claim 1 wherein the magnitude of saidoutput current is controllable by controlling the resistance of saidsecond resistor.

3. A current source for providing an output current which issubstantially independent of temperature variations, said sourcecomprising:

a. a first and a second transistor, said transistors being of the sameconductivity type and having substantially equal alphas and betas;

b. said first transistor having its base connected directly to thecollector of said second transistor;

c. said first transistor having its emitter connected directly to thebase of said second transistor;

. a source of regulated voltage;

e. a first resistor connected between one of the terminals of saidvoltage source and the collector of said second transistor;

a second resistor connected between the junction point of the emitter ofsaid first transistor and the base of said second transistor, and theother terminal of said voltage source, the emitter of said secondtransistor being coupled to said other terminal of said voltage source;

g. a third resistor connected between the emitter of said secondtransistor and the other terminal of said voltage source; and

h. means isolated from said source and connected to the collector ofsaid first transistor for deriving an output current, said outputcurrent being substantially independent of temperature provided that thereciprocal value of the beta of each of said transistors issubstantially negligible compared to one, and that the resistance ofsaid first and third resistors are substantially zero.

1. A current source for providing an output current which is linearlydependent upon temperature variations, said source comprising: a. afirst and a second transistor, said transistors being of the sameconductivity type and having substantially equal alphas and betas; b.said first transistor having its base connected directly to thecollector of said second transistor; c. said first transistor having itsemitter connected directly to the base of said second transistor; d. asource of regulated voltage; e. a first resistor connected between oneof the terminals of said voltage source and the collector of said secondtransistor; f. a second resistor connected between the junction point ofthe emitter of said first transistor and the base of said secondtransistor, and the other terminal of said voltage source, the emitterof said second transistor being coupled to said other terminal of saidvoltage source; and g. means isolated from said source and connected tothe collector of said first transistor for deriving an output current,said output current being substantially a linear function of temperatureprovided the reciprocal value of the beta of each of said transistors issubstantially negligible compared to one.
 2. A current source as definedin claim 1 wherein the magnitude of said output current is controllableby controlling the resistance of said second resistor.
 3. A currentsource for providing an output current which is substantiallyindependent of temperature variations, said source comprising: a. afirst and a second transistor, said transistors being of the sameconductivity type and having substantially equal alphas and betas; b.said first transistor having its base connected directly to thecollector of said second transistor; c. said first transistor having itsemitter connected directly to the base of said second transistor; d. asource of regulated voltage; e. a first resistor connected between oneof the terminals of said voltage source and the collector of said secondtransistor; f. a second resistor connected between the junction point ofthe emitter of said first transistor and the base of said secondtransistor, and the other terminal of said voltage source, the emitterof said second transistor being coupLed to said other terminal of saidvoltage source; g. a third resistor connected between the emitter ofsaid second transistor and the other terminal of said voltage source;and h. means isolated from said source and connected to the collector ofsaid first transistor for deriving an output current, said outputcurrent being substantially independent of temperature provided that thereciprocal value of the beta of each of said transistors issubstantially negligible compared to one, and that the resistance ofsaid first and third resistors are substantially zero.